This invention relates to high temperature superconductors (HTS), and more particularly to superconducting layers and methods of depositing precursor compositions for such layers.
Coated conductors, comprising a single or multiple combinations of a biaxially textured high temperature superconductor (xe2x80x9cHTSxe2x80x9d) layer on a thin buffer layer and a substrate tape, are a cost-performance-effective technology for manufacturing long length flexible HTS wire for magnet, coil and power applications. For example, these conductors should be useful for power transmission cables, rotor coils of motors and generators, and windings of transformers, as well as for magnets for medical magnetic resonance imaging (MRI), magnetic separation, ion-beam steering and magnetic levitation. Particularly of interest here are applications which use ac currents and fields, or fast ramps of current and field, for example ac power transmission cables, transformers, faultcurrent limiters, magnetic separation magnets and high energy physics magnets.
Some background on biaxially textured high temperature superconducting xe2x80x9ccoated conductorsxe2x80x9d is known. Such coated conductors include at least, for example, a substrate and a superconducting layer (such as YBCO) deposited thereon. One or more buffer layers may be included between the substrate and the superconductor material. An advantage of such materials as YBCO (YBa2Cu3Ox, or Yttrium-Barium-Copper-Oxide) films is the very high critical current densities attainable, particularly in magnetic fields. Other related superconducting materials which can be used are REBa2Cu3Ox, where the Y has been partially or completely replaced by rare earth elements (RE). As information as to the requirements for commercial application, and limitations on conductor technology has become available, the potential for low production costs of these rare-earth superconducting materials (including YBCO) has also become of interest in further development.
Certain challenges in this field include the need for cost effective methods for producing chemically compatible biaxially textured buffer layers, as well as the need to deposit sufficient thickness of the high critical current density superconducting layer. Regarding the first objective, it appears that deformation textured substrates with epitaxial buffer layers can be made cost effective. In addition ion beam assisted deposition of thin MgO layers with epitaxial top layers may prove to be economically viable.
Regarding the need to deposit thick layers of superconductor precursor compositions, a number of techniques have been evaluated. Chemical vapor deposition (CVD) is not considered a competitive method at this time, due to the very high cost of precursor materials. Most physical vapor deposition (PVD) methods, (for example, pulsed laser ablation, reactive sputtering and electron beam evaporation) are limited by deposition rate, compositional control, and high capital costs. A possible economical PVD method would be thermal or electron beam evaporation of the rare earth elements, copper and barium fluoride, known as the xe2x80x9cbarium fluoridexe2x80x9d process. This process appears to be more rapid than direct PVD methods, but capital costs and control system costs are still likely to be too high. Additionally, the deposited precursor composition must subsequently be reacted in a separate furnace system to form the HTS film.
Solution deposition methods have been evaluated, and these appear to offer much lower costs, since vacuum systems are eliminated. Thus, capital costs are not as high, and deposition rates not as low, as other methods using vacuum systems. Trifluoroacetate (TFA) solution processes offer low costs for precursor compositions, high deposition rate, and non-vacuum processing advantages. Such processes are described, for example, in U.S. Pat. No. 5,231,074 to Cima et al., and PCT Publication No. WO 98/58415, published Dec. 23, 1998 and require dissolution of the constituents of the precursor composition to form a solution phase. Both U.S. Pat. No. 5,231,074 and PCT Publication No. WO 98/58415 are hereby incorporated by reference in their entirety.
For commercial processes, it is desirable to have a composition serving as a precursor to superconducting films, which can be coated onto large area substrates in a single application using high-deposition rate, to produce a desired film thickness. The precursor composition is preferably convertible to the superconducting phase by way of simple thermal processes.
The invention provides a low cost method for fabricating thick film precursor compositions of rare-earth superconductors on long lengths of substrate. The final thicknesses of such films are preferably between about 1 micron and about 5 microns. The specific superconductors of interest are high temperature superconductors of the class of rare-earth barium cuprate species (REBCO), including, for example, YBa2Cu3O6.8 (YBCO), or systems based on thallium/barium/calcium/copper/oxide (ThBCCO) or bismuth/strontium/calcium/copper/oxide (BSCCO), and other known superconducting materials, including versions doped with other species. Of particular interest are those materials having superconducting transition temperatures, Tc, above about 77 K. The most useful buffered substrates for such films are biaxially textured, providing an epitaxial growth template for achieving maximum attainable critical current densities (Jc). The precursor compositions for the superconducting layer of such high temperature superconductors include solid-state, or semi solid-state precursors deposited in the form of a dispersion. These precursor compositions allow for example the substantial elimination of in the case of YBCO BaCO3 formation in final YBCO superconducting layers, while also allowing control of film nucleation and growth.
As used herein, xe2x80x9cbiaxially texturedxe2x80x9d refers to a surface for which the crystal grains are in close alignment with a direction in the plane of the surface. One type of biaxially textured surface is a cube textured surface, in which the crystal grains are also in close alignment with a direction perpendicular to the surface. Examples of cube textured surfaces include the (100)[001] and (100)[011] surfaces, and an example of a biaxially textured surface is the (113)[211] surface. As used herein, xe2x80x9cepitaxial layerxe2x80x9d refers to a layer of material, the crystallographic orientation of which is directly related to the crystallographic orientation of the surface of a layer of material onto which the epitaxial layer is deposited. For example, for a multi-layer superconductor having an epitaxial layer of superconductor material deposited onto a substrate, the crystallographic orientation of the layer of superconductor material is directly related to the crystallographic orientation of the substrate. Thus, in addition to the above-discussed properties of a substrate, it can also be desirable for a substrate to have a biaxially textured surface or a cube textured surface.
As used herein, a xe2x80x9cdispersionxe2x80x9d is a two-phase system in which one phase includes finely divided particles distributed throughout a liquid second phase. As used herein, xe2x80x9cultrafine particlesxe2x80x9d are those particles sufficiently small to allow a uniform distribution of cation elements within the precursor composition, and a chemically homogeneous superconducting film. For a superconducting film of from about 1 to about 10 microns thickness, the particle diameters are typically less than about 10% of the final film thickness. In addition, the particle sizes are small enough, and uniformly distributed enough, to allow rapid local diffusion of cationic constituents of the precursor compositions, for the efficient formation of substantially stoichiometric superconducting layers. As used herein, xe2x80x9csubstantially stoichiometricxe2x80x9d refers to the elemental ratios in mixtures of materials, in which the atomic ratios of cationic elements are within about 10% of whole number values. Such deviations from whole number stoichiometries can be deliberately introduced or can arise from the supply of materials, and can be desirable to aid in material processing. Experiments can show that excess elements are typically rejected by the material as a whole, resulting in nearly stoichiometric amounts of the elements.
The invention allows the deposition of thick films in a single deposition step. Although multiple deposition steps can be carried out according to the particular application, each of said steps can give a film thicker than that available from previously known solution processes. Simplified binder removal or decomposition may significantly improve the prospect of a low cost superconductor preparation method.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.
Other features and advantages of the invention will be apparent from the following detailed description, and from the claims.